This invention relates to lasers and, more particularly, to arrays of lasers.
Single element tunable lasers have been created using micromachined devices, temperature change or electron injection tuning. However, all such wavelength tuning techniques are xe2x80x9canalogxe2x80x9d tuning techniques. All of the methods change the optical length of the laser cavity, which affects the resonant wavelength in the laser. This change in laser cavity length is typically achieved in one of two ways.
One way the physical length of the cavity can be changed is shown in FIG. 1. FIG. 1 is an example of a mechanically tunable vertical cavity surface emitting laser 100 (VCSEL) of the prior art. With a VCSEL 100 of FIG. 1, tuning is performed by either using a micromachined mirror 102 fixed on the laser substrate, which can be moved up and down (i.e. closer or farther relative to the cavity), or by using an external mirror separate from the laser that can be physically moved, so the cavity length can be physically changed. To tune a laser with a micromachine however, such as shown in FIG. 1, requires up to 100 Volts to move the mirror. For an externally tuned laser, a separate mechanical or piezoelectric motor must be used, thereby requiring large voltages as well.
The other way to change the cavity length is to change the perceived length of the device, for example, by changing the refractive index of the material, which affects the speed of light in the material and hence its wavelength.
FIG. 2 is an example of a temperature tunable distributed feedback (DFB) 200 laser of the prior art. Temperature tuning is done by adding additional contacts onto the laser and heating the laser material 202, such as shown in FIG. 2, which changes its wavelength. Another way to change the refractive index is by injecting extra electrons into the structure, which creates a carrier induced index change. Thermal or injection tuning, however, requires large currents (that are not used for lasing) to be put into the device structure which significantly impacts power usage.
While changing the wavelength of a laser can be intentionally accomplished, because changing the temperature changes the wavelength, such temperature based wavelength changes, when unintentional, can be detrimental. Thus, in cases where a very accurate wavelength is required from a fixed wavelength laser, the laser must have some form of active temperature compensation (for example by using a thermoelectric cooler) to adjust for wavelength drift caused by the temperature change.
In other tunable lasers, cavity length (actual or perceived) tuning can be used to compensate for temperature drift, but the tuning mechanism takes significant power.
Moreover, the equipment required to compensate for temperature drift is large, bulky and expensive, in both material cost and power usage.
Thus, all of these analog tuning methods lack precision and controllability and/or require high power. Moreover, lasers of the prior art can not easily switch between variable wavelength applications and rigorously fixed wavelength ones.
We have devised a way to create multi-wavelength lasers that are more precise than the multi-wavelength lasers available in the prior art.
We have also devised a way to create multi-wavelength lasers that are more controllable than the multi-wavelength lasers available in the prior art.
We have further devised a way to create multi-wavelength lasers that require less power than the multi-wavelength lasers in the prior art.
We have also devised a way to create multi-wavelength lasers that are xe2x80x9cdigitallyxe2x80x9d tuned as opposed to the analog tuning used with multi-wavelength lasers in the prior art.
We have further devised a way to use a single semiconductor laser array for both variable and precision fixed wavelength applications.
By applying the teachings of the invention, multi-wavelength laser arrays can be created that are useable as wavelength routers, as switches, as xe2x80x9cdigitallyxe2x80x9d tunable sources, or as a digitally controllable stable wavelength source.
Moreover, by applying the teachings of the invention, no additional power is needed for tuning, so power (and applied current) can be used for sending data to the laser. The power requirements do not change based upon of which laser (and thus which wavelength) is selected or because of temperature induced wavelength drift.
The advantages and features described herein are a few of the many advantages and features available from representative embodiments and are presented only to assist in understanding the invention. It should be understood that they are not to be considered limitations on the invention as defined by the claims, or limitations on equivalents to the claims. For instance, some of these advantages are mutually contradictory, in that they cannot be simultaneously present in a single embodiment. Similarly, some advantages are applicable to one aspect of the invention, and inapplicable to others. Thus, this summary of features and advantages should not be considered dispositive in determining equivalence. Additional features and advantages of the invention will become apparent in the following description, from the drawings, and from the claims.